1. Field of the Invention
The invention relates to a constant voltage output circuit and, more particularly, to a constant voltage output circuit which can reduce a restriction in a manufacturing process and can obtain a wide voltage set range.
2. Related Background Art
Hitherto, particularly, in an electronic circuit which handles an analog signal, there is a case where in addition to a ground level (ground) and a power voltage, a constant intermediate voltage source which is not susceptible to a variation in power of a power source and temperature is needed.
FIG. 1 is a diagram showing an example of a conventional constant voltage output circuit. In the diagram, reference numeral 1 denotes a bipolar transistor (hereinafter, abbreviated to BJT); 2 indicates a BJT whose size is larger than the BJT 1. The size of BJT 2 is generally just an integer value times as large as the size of BJT 1. Reference numerals 3 and 4 denote resistors having a same resistance value R.sub.0. Terminals 5 and 6 of the resistors 3 and 4 are connected to collector terminals of the BJT 1 and BJT 2, respectively. The other terminals of the resistors 3 and 4 are mutually connected and become a common terminal 7. Reference numeral 8 denotes a resistor of a resistance value R.sub.1 connecting an emitter of the BJT 2 and ground and 9 indicates an operational amplifier (hereinafter, referred to as an op-amplifier) in which a (+) input terminal (non-inverting input terminal) is connected to the terminal 5, a (-) input terminal (inverting input terminal) is connected to the terminal 6, and an output is connected to the common terminal 7. An emitter of the BJT 1 is directly connected to ground. Bases of the BJTs 1 and 2 are mutually connected to the terminal 5.
FIG. 2 shows a constructional example of the BJT 2. Collectors of four BJTs 1' of the same size as that of the BJT 1 are mutually connected, their bases are mutually connected, and their emitters are mutually connected, thereby setting the size of BJT 2 to be just four times as large as that of BJT 1.
In the circuit of FIG. 1, a point will now be described that by setting resistance values R.sub.0 and R.sub.1 in accordance with characteristics of the BJTs 1 and 2, a predetermined voltage can be generated from the terminal 7. It is now assumed that the size of BJT 2 is four times as large as that of BJT 1 and current gain of the BJT 2 is large and a emitter current and a collector current are equal.
In FIG. 1, current flowing through the resistor 3, namely, a collector current of the BJT 1 is labeled as 1.sub.0. Since electric potentials of the terminals 5 and 6 are equal due to the operation of the operational amplifier 9, a current flowing in the resistor 4, namely, the collector current of the BJT 2 is also equal to I.sub.0. Now, assuming that the output voltage of the terminal 7 is called V.sub.BG and base-emitter voltages of the BJTs 1 and 2 are set to V.sub.BE1 and V.sub.BE2, EQU V.sub.BG =V.sub.BE1 +I.sub.0 R.sub.0 (1) EQU V.sub.BE1 =V.sub.BE2 +I.sub.0 R.sub.1 (2)
are satisfied. Since the size of BJT 2 is four times as large as that of the BJT 1, EQU V.sub.BE1 -V.sub.BE2 =(kT/q).multidot.In.sub.4 (3)
is satisfied.
Where,
k: Boltzmann's constant
T: absolute temperature
q: unit charges
By deleting V.sub.BE2 and I.sub.0 from the equations (1), (2), and (3), we have EQU V.sub.BG =V.sub.BE1 +(R.sub.0 /R.sub.1).multidot.(kT/q).multidot.In.sub.4 ( 4)
By differentiating both sides of the equation (4) by T, EQU dVB.sub.BG /dT=dV.sub.BE1 /dT+(R.sub.0 /R.sub.2)*(k/q).multidot.In.sub.4 ( 5)
is satisfied.
By deciding R.sub.0 /R.sub.1 so as to obtain EQU dV.sub.BE1 /dT+(R.sub.0 /R.sub.1).multidot.(k/q).multidot.In.sub.4 =0
in accordance with the temperature characteristics of the BJT, the temperature dependency of V.sub.BG is eliminated from the equation (5). In the ordinary silicon BJT, since dV.sub.BE1 /dT is equal to about -2mV/K, R.sub.0 /R.sub.1 is equal to about 16. Generally, since the values of R.sub.0 and R.sub.1 are determined so that V.sub.BE1 is equal to about 0.6V, the value of V.sub.BG is equal to about 1.2V as will be understood from the equation (4).
As described above, by setting the values of R.sub.0 and R.sub.1 in accordance with the BJT characteristics, a predetermined output voltage is derived from the terminal 7. By using such voltage as a reference for the electronic circuit, a voltage level can be accurately set.
In the above example, however, a BJT in which an emitter, a base, and a collector can be taken out as independent terminals is necessary. Although the constant voltage output circuit is often used in a semiconductor IC, the above example can only be applied in a manufacturing process such that the BJT as mentioned above can be formed. There is a problem such that the above example cannot be applied to an IC using a manufacturing process which cannot form an independent BJT.
An output of the op-amplifier which is used in the above example is also a collector current source of the BJT and it is necessary to use an op-amplifier having a high current supplying ability. There is inevitably a problem such that the size of the op-amplifier has to be enlarged.
Further, as will be understood from the equation (4) of the constant voltage output, V.sub.BG can be changed by selecting the set potentials of V.sub.BE1. However, generally, since a range where normal current-voltage characteristics of the bipolar transistor can be held is a range of about 0.5 to 0.7V as V.sub.BE1, the constant voltage output in only a range of about 1.1 to 1.3V can be also set. In other words, there is a problem such that a selection width of the constant voltage output value is narrow.